1. Field of the Invention
The present invention relates to a screw-rotating tool to be used to tighten or loosen screw parts such as bolts, nuts, and the like.
2. Description of the Related Art
A tightening means using screw parts such as bolts, nuts, and the like is widely used nowadays because it is convenient and secures strength. Although spanners, monkey wrenches, and the like are used to tighten or loosen the screw parts, a ratchet tool incorporating a ratchet mechanism is superior to them because the ratchet tool can be used conveniently.
As shown in FIG. 10, in the ratchet tool, a handle (a) is pivoted within a predetermined angle. As a result, a click (b) rotates a ratchet wheel (c), and a socket (d) integral with the ratchet wheel (c) can be rotated in one direction. An engaging portion (e) of the ratchet tool engaging a screw part is formed on the inner peripheral surface of the socket (d). An urging spring (f) is installed proximately to the click (b). Accordingly, it is possible to tighten or loosen the screw part easily by engaging the socket (d) with the screw part only once in the socket (d), unlike a spanner which requires a worker to engage the socket (d) with the screw part repeatedly.
However, the ratchet tool has a limited operable range: When the resistance between a female screw and a bolt becomes smaller than that between the ratchet wheel and the click in loosening the bolt, the bolt is rotated even by reverse pivotal motions (tightening direction) of the handle, with the ratchet mechanism being unoperative. A similar situation occurs from an initial stage to an intermediate stage in a bolt-tightening operation. Thus, the ratchet tool is not useful except the final stage in the bolt-tightening operation.
In order to overcome such a problem, there is disclosed in Japanese Laid-Open Patent Publication No. 8141921 an art of installing a hand ring on the periphery of the socket of the ratchet tool. The hand ring is rotated to tighten or loosen the bolt when the resistance between the female screw and the bolt becomes small. The operation of rotating the hand ring is troublesome.